There is an increasing demand for wideband antennas for use within wireless communication, in order to allow communication in several frequency bands, the use of high or very high data rates and for different systems. Ultra Wide Band (UWB) signals are generally defined as signals having a large relative bandwidth (bandwidth divided by carrier frequency) or a large absolute bandwidth. The expression UWB is particularly used for the frequency band 3.2-10.6 GHz, but also for other and wider frequency bands.
The use of wideband signals is for example described in “History and applications of UWB”, y M. Z. Win et. al, Proceedings of the IEEE, vol. 97, No. 2, p. 198-204, February 2009.
UWB-technology is a low cost technology. Development of CMOS processors transmitting and receiving UWB-signals has opened up for a large field of different applications and they can be fabricated at a very low cost for UWB-signals without requiring any hardware for mixers, RF (Radio Frequency)-oscillators or PLLs (Phase Locked Loops).
UWB technology can be implemented in a wide range of areas, for different applications, such as for example short range communication (less than 10 m) with very high data rates (up to or above 500 Mbps), e.g. for wireless USB similar communication between components in entertainment systems such as DVD players, TV and similar; in sensor networks where low data rate communication is combined with precise ranging and geolocation, and radar systems with extremely high spatial resolution and obstacle penetration capabilities, and generally for wireless communication devices.
To generate, transmit, receive and process UWB signals, the development of new techniques and arrangements within the fields of generation of signals, signal transmission, signal propagation, signal processing and system architectures is required.
Generally UWB antennas have been divided into four different categories of which the first category, the scaled category, comprises bowtie dipoles, see for example “A modified Bow-Tie antenna for improved pulse radiation”, by Lestari et.al, IEEE Trans. Antennas Propag., Vol. 58, No. 7, pp. 2184-2192, July 2010, biconical dipoles as for example discussed in “Miniaturization of the biconical Antenna for ultra-wideband applications” by A. K. Amert et. al, IEEE Trans. Antennas Propag., Vol. 57, No. 12, pp. 3728-3735, December 2009. The second category comprises self-complementary structures as e.g. described in “Self-complementary antennas” by Y. Mushiake, IEEE Antennas Propag. Mag., vol. 34, No. 6, pp. 23-29, December 1992. The third category comprises travelling wave structure antennas, e.g. the Vivaldi antenna as e.g. discussed in “The Vivaldi aerial” by P. J. Gibson, Proc. 9th European Microwave conference, pp. 101-105, 1979, and the fourth category comprises multiple resonance antennas like log-periodic dipole antenna arrays.
Antennas from the scaled category, the self-complementary category and the multiple reflection category comprise compact, low profile antennas with low gain, i.e. having wide and often more or less omni-directional far field patterns, whereas antennas of the travelling wave category, like the Vivaldi antennas, are directional.
The above-mentioned UWB antennas were mainly designed for use in normal Line-of-Sight (LOS) antenna systems with one port per polarization and a known direction of the single wave between the transmitting and receiving side of the communication system. In most environments, however, there are several objects (such as houses, trees, vehicles, humans) between the transmitting and receiving sides of the communication systems that cause reflections and scattering of the waves, resulting in a multiple of incoming waves on the receiving side, which has as a consequence that there was a need for antennas better accounting for these factors. Interference between these waves causes large level variations known as fading of the received voltage (known as the channel) at the port of the receiving antenna. This fading can be counteracted in modern digital communication systems making use of multiport antennas and support MIMO technology (multiple-input multiple-output).
Wireless communication systems may comprise a large number of micro base stations with multiband multiport antennas enabling MIMO with high requirements as to compactness, angular coverage, radiation efficiency and polarization schemes, which all are critical issues for the performance of such systems. The radiation efficiency of a multiport antenna is reduced by ohmic losses and impedance mismatch like in single-port antennas, but also by mutual coupling between the antenna ports.
Earlier known wideband antenna arrangements did not satisfactorily meet these requirements.
In WO2014/062112, though, a wideband compact multiport antenna suitable for MIMO communication systems as described above is disclosed, which has low ohmic losses, i.e. high radiation efficiency, good matching as well as low coupling between antenna ports. The geometry shown in FIG. 11 of WO2014/062112 is known as a dual-polarized self-grounded bowtie antenna, and is described in H. Raza, A. Hussain, J. Yang and P.-S. Kildal, “Wideband Compact 4-port Dual Polarized Self-grounded Bowtie Antenna”, IEEE Transactions on Antennas and Propagation, Vol. 62, No., pp. 1-7, September 2014. The geometry of the self-grounded bowtie antenna is expensive to manufacture in large volumes, and in particular to mass produce.
For future wireless communication systems, such as e.g. the fifth wireless generation (5G), the frequencies used may be up to 30 GHz, or even up to 60 GHz, and Massive MIMO is a challenging option for providing a sufficient gain and steer-ability at millimeter wave frequencies, see “Preparing for GBit/s Coverage in 5G: Massive MIMO, PMC Packaging by Gap Waveguides, OTA Testing in Random LOS” by Per-Simon Kildal, 2015 Loughborough Antennas & Propagation Conference, 2 & 3 Nov. 2015.
Massive MIMO array antennas, or Large-scale Antenna Systems or Very Large MIMO arrays etc. are, contrarily to hitherto known antenna systems, based on the use of a large number of antenna elements, from a few tenths to hundreds or even thousands thereof, for being operated independently to adapt coherently to the incoming wave or waves in the environments in such a way that the signal-to-noise ratio is maximized. Massive MIMO is particularly advantageous in that data throughput and energy efficiency can be considerably increased e.g. when a large number of user stations are scheduled simultaneously, i.e. a multi-user scenario.
MIMO arrays and Massive MIMO Array antennas consist of several equal antenna elements side by side. This makes manufacture as well as and mounting extremely difficult, expensive and time consuming.
A massive MIMO array is the digital equivalent to a traditional phased array antenna. The phased array contains analogue controllable phase shifters on all elements in order to phase-steer the antenna beam to the direction needed. In MIMO technology there is an Analogue to Digital Converter (ADC) or a Digital to Analogue Converters (DAC) on each element, so that all beam-steering is done digitally, and no analogue phase shifters are needed. This makes the MIMO antenna system much more flexible and adaptive than phased-arrays, so that any beam shape and even multiple beams can be formed. This is referred to as digital beam-forming.
All known antenna arrangements, even if meeting many of the functional requirements referred to above, suffer from the drawbacks of not being sufficiently easy and cheap to fabricate and not being as easy to mount as would be desired. This is a problem both for older and present generations of communication systems, and also for other implementations, but become even more pronounced for future communication systems, such as e.g. 5G, and also other future applications at higher frequencies than those used today. They also suffer from the drawback of not providing a sufficient bandwidth.